The impact that an efficient, inexpensive and reliable visible laser source would have on data storage, display technology, undersea communications and optical processing has provided the stimulus for much recent work on solid state visible lasers. One approach that has yielded much success is the use of upconversion processes within rare earth doped materials to produce laser emission at a wavelength significantly shorter than the pump wavelength. Applied to bulk crystals, lasing has been demonstrated at 380 nm in Nd:LaF.sub.7 and 551 nm and 671 nm in Er:YLiF.sub.4. (See MACFARLANE, R. M. , TONG F. , SILVERSMITH, A. J. , AND LENTH, W.: `Violet cw neodymium upconversion laser`, Appl. Phys. Lett., 1988, 52, (16), pp. 1300-1302, and MACFARLANE, R. M.: `Dual wavelength visible upconversion laser`, Appl Phys. Lett., 1989, 54, (23), pp. 2301-2302, respectively). Unfortunately all of these schemes require liquid nitrogen cooling. However, the recent demonstration of visible lasing at 480 nm in Tm.sup.3+ -doped fluoride fibre (see ALLAIN, J. Y., MONERIE, M AND POIGNANT, H.: `Blue upconversion fluorozirconate fibre laser`, Electron. Lett., 1990, 26 (3), pp. 166-168) coupled with the demonstration of room temperature lasing at red, green and blue wavelengths in praseodymium doped fluorozirconate glass fibre (see SMART, R. G. , HANNA, D. C., TROPPER, A. C. , DAVEY, S. T. , CARTER, S. F. , SZEBESTA, D,: `CW upconversion lasing at blue, green and red wavelengths in an infrared-pumped Pr.sup.3+ -doped fluoride fibre at room temperature`, PD paper, CLEO 91, Baltimore, Md., USA (also published in Electron. Lett., 1991, 27, (14), pp 1307-1309) has dramatically changed the viability of such upconversion pumped laser schemes.
The applicants have determined that significant gain at 850 nm can be achieved when erbium doped fluorozirconate fibre is pumped at 800 nm. The 850 nm emission they observed originated from the .sup.4 S.sub.3/2 level, and required the sequential absorption of 800 nm pump photons. The present invention is based on the observation by the applicants that there is also green fluorescence which is ascribed to a transition between the .sup.4 S.sup.3/2 level and the .sup.4 I.sub.15/2 ground state of the system (FIG. 1). In bulk crystals doped with erbium (Er: YLiF.sub.4) this is a well known transition and has resulted in upconversion lasing at 551 nm when cooled to 77K. (See McFarlane ibid--Appl. Phys. Lett 54 (23) )
In such bulk crystal lasers, ions are excited into the upper laser level predominantly through an ion-ion interaction between ions residing in the .sup.4 I.sub.11/12 manifold. However, in lightly doped fluorozirconate fibre, ion-ion interactions are negligible, and the dominant excitation mechanism is attributed by the applicant to excited state absorption (ESA) of pump photons.
FIG. 1 represents the energy levels of the trivalent erbium ion, with the relevant green lasing transition indicated between the .sup.4 S.sub.3/2 level and the .sup.4 I.sub.15/2 ground state The upper laser level may be populated by the sequential absorption of 801 nm pump photons in a process which involves excitation of ground state ions into the .sup.4 I.sub.9/2 band some of which then branch into the .sup.4 I.sub.11/2 and the .sup.4 I.sub.13/2 levels. These ions are then further excited by pump photons into the .sup.4 S.sub.3/2 and .sup.2 H.sub.11/2 and other levels. A large proportion of the ions in these higher energy levels then relax into the .sup.4 S.sub.3/2 upper laser level, from which a direct transition to the ground state is responsible for the green emission.
According to a first aspect of the present invention an optical amplifier comprises: a fluorozirconate waveguide doped with erbium ions; and an optical pump means coupled to said waveguide for providing an optical pump signal capable of exciting said erbium ions into the .sup.4 S.sub.3/2 energy level; whereby the amplifier can provide optical gain at about 546 nm.
The waveguide may conveniently comprise a fluorozirconate optical fibre waveguide but other types of waveguide may be employed. For example, it is expected that a useful configuration would be a planar waveguide structure formed by doping a fluorozirconate glass substrate. High dopant concentration would lead to compact (short waveguide length ) devices.
The invention has been demonstrated in standard ZBLAN fluorozirconate waveguides; the proportions of the glass components is not expected to be critical.
The pump may be coupled to the fibre by any known appropriate technique. The high numerical apperture fluorozirconate fibre may be spliced to a silica fibre so that readily available fused couplers, for example, can be used to couple pump and signal sources to the doped fibre.
The .sup.4 S.sub.3/2 energy level of the erbium ions can be excited by pump wavelengths in the range 791 nm to 812 nm, preferably 801 nm.
According to a second aspect of the present invention, a laser comprises: a fluorozirconate waveguide doped with erbium ions; an optical pump means coupled to said waveguide for providing an optical pump signal capable of exciting said erbium ions into the .sup.4 S.sub.3/2 energy level; and a pair of reflectors, one at each end of the waveguide, which reflectors define a resonant cavity, the reflectors having reflectivities to provide lasing action at about 546 nm when the fibre is pumped by the pumping means.
The reflectors, which may be mirrors or other reflectors such as Sagnac loop reflectors, define a Fabry-Perot cavity and in known manner are selected to provide reflections sufficient to sustain lasing only at the desired wavelength.
When pumped at 801 nm the green emission suffers from competition with the 4-level 850 nm transition. The applicants have determined that for lasing to occur on this .sup.4 S.sub.3/2 -.sup.4 I.sub.13/2 transition it is necessary to pump ions out of the, long lived, .sup.4 I.sub.13/2 terminal state. In the 801 nm pumped scheme this condition is met through the strong pump ESA out of .sup.4 I.sub.13/2 state.
It has been found preferable to use a pump wavelength of about 971 nm. In this 971 nm pumping scheme a population is allowed to build up in the .sup.4 I.sub.13/2 level and thus reduce the likelihood of transitions at 850 nm.
The dopant concentrations useful with the present invention is governed by certain general criteria applicable to doped waveguide amplifiers. For low dopant levels of about 0.01 wt % the length of the doped waveguide must be long enough to provide the required gain, but this competes with the fibre background loss of about 0.2 dB/m which effectively diminishes the amplifier or laser performance. To optimise the length-density product, values of concentration of 0.1 wt % are typically chosen. At a concentration of about 1 wt % various ion-ion interactions occur which degrade the amplifier or laser performance. At concentrations in excess of about 4wt % the glass becomes unstable and unusable. These considerations are well known and the level of dopant required in a given application can be readily determined.
On the basis of the above criteria waveguide lengths will generally be in the range 0.1 to 10 m with lengths in the range 1 to 5 m being prefered.